Systems for protecting internal components of an EUV light source from plasma-generated debris

ABSTRACT

Systems and methods are disclosed for protecting an EUV light source plasma production chamber optical element surface from debris generated by plasma formation. In one aspect of an embodiment of the present invention, a shield is disclosed which comprises at least one hollow tube positioned between the optical element and a plasma formation site. The tube is oriented to capture debris while allowing light to pass through the tube&#39;s lumen via reflection at relatively small angles of grazing incidence. In another aspect of an embodiment of the present invention, a shield is disclosed which is heated to a temperature sufficient to remove one or more species of debris material that has deposited on the shield. In yet another aspect of an embodiment of the present invention, a system is disclosed which a shield is moved from a light source plasma chamber to a cleaning chamber where the shield is cleaned.

RELATED APPLICATIONS

This application is a divisional of U.S. Patent application Ser. No. 11/067,099 filed on Feb. 25, 2005. The present application is related to U.S. Pat. application Ser. No. 10/979,945wntitled LPP EUV LIGHT SOURCE, filed on Nov. 1, 2004, Attorney Docket No. 2004-0088-01, and Ser. No. 10/900,839, entitled EUV LIGHT SOURCE, filed on Jul. 27, 2004, Attorney Docket No. 2004-0044-01, and Ser. No. 10/803,526, entitled HIGH REPETITION RATE LPP EUV LIGHT SOURCE, filed on Mar. 17, 2004, Attorney Docket No. 2003-0125-01 and Ser. No. 10/798,740, entitled COLLECTOR FOR EUV LIGHT, filed on Mar. 10, 2004, Attorney Docket No. 2003-0083-01, the disclosures of each of which is incorporated by reference.

FIELD OF THE INVENTION

The present invention relates to extreme ultraviolet (“EUV”) light generators providing EUV light from a plasma created from a source material and collected and directed to a focus for utilization outside of the EUV light source generation chamber, e.g., for semiconductor integrated circuit manufacturing photolithography process performing machines, e.g., at wavelengths of around 20 nm and below.

BACKGROUND OF THE INVENTION

Extreme ultraviolet (“EUV”) light, e.g., electromagnetic radiation having wavelengths of around 20 nm or less (also sometimes referred to a soft x-rays), and including light at a wavelength of about 13.5 nm, can be used in photolithography processes to produce extremely small features in substrates, e.g., silicon wafers.

Methods to produce EUV light include, but are not necessarily limited to, converting a material into a plasma state that has an element, e.g., xenon, lithium or tin, with an emission line in the EUV range. In one such method, often termed electric discharge produced plasma (“DPP”), the plasma may be produced by an electrical discharge between a pair of electrodes. In another method, the required plasma can be produced by irradiating a target material, such as a droplet, stream or cluster of material having the required line-emitting element, with a laser beam. This later process is referred to as laser produced plasma (“LPP”).

For each of these processes, the plasma is typically produced in a sealed vessel, e.g., vacuum chamber, and monitored using various types of metrology equipment. In addition to generating EUV radiation, these plasma processes also typically generate undesirable by-products in the plasma chamber which can include heat, high energy ions and scattered debris from the plasma formation, e.g., atoms and/or clumps of source material that is not fully ionized in the plasma formation process.

These plasma formation by-products can potentially damage or reduce the operational efficiency of the various plasma chamber optical elements including, but not limited to, the surfaces of metrology detectors, windows used to image the plasma formation process, and in the case of LPP, the laser input window. The heat, high energy ions and/or source material debris may be damaging to the optical elements in a number of ways, including heating them, coating them with materials which reduce light transmission, penetrating into them and, e.g., damaging structural integrity and/or optical properties, e.g., the ability of a mirror to reflect light at such short wavelengths, corroding or eroding them and/or diffusing into them. In addition, some optical elements, e.g., the laser input window, form a part of the vacuum chamber and are thus placed under a stress when a vacuum is present in the plasma chamber. For these elements, deposits and heat can combine to fracture, (i.e., crack) the element resulting in a loss of vacuum and requiring a costly repair.

Accessing contaminated or damaged optical elements in the plasma chamber for the purpose of cleaning or replacing the elements can be expensive, labor intensive and time-consuming. In particular, these systems typically require a rather complicated and time consuming purging and vacuum pump-down of the plasma chamber prior to a re-start after the plasma chamber has been opened. This lengthy process can adversely affect production schedules and decrease the overall efficiency of light sources for which it is typically desirable to operate with little or no downtime.

With the above in mind, Applicants disclose shielding systems and methods useful for increasing the service life of various plasma chamber optical elements and for increasing the time between maintenance operations that require opening of the plasma chamber.

SUMMARY OF THE INVENTION

Systems and methods are disclosed for protecting an EUV light source plasma production chamber optical element surface from debris generated by plasma formation. In one aspect of an embodiment of the present invention, a light transmitting shield is disclosed which is movable into a cleaning chamber from an initial position where the shield is interposed between a plasma formation site and the optical element. In the initial position, the shield prevents debris directed toward the optical element surface from reaching the surface.

In another aspect of an embodiment of the present invention, a shield for a plasma chamber optical element is disclosed which may be heated to a temperature sufficient to remove one or more species of debris material that has deposited on the shield.

In still another aspect of an embodiment of the present invention, a shield system for a plasma chamber optical element is disclosed which comprises at least one hollow tube positioned in between the optical element and a plasma formation site. The tube is oriented to capture debris while allowing light to pass through the lumen, (i.e., bore) of the tube via reflection at relatively small angles of grazing incidence.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a schematic view of an overall broad conception for a laser-produced plasma EUV light source according to an aspect of the present invention;

FIG. 2 shows in schematic, cross-sectional form a possible embodiment of a system for protecting an optical element surface from plasma-source debris according to an aspect of an embodiment of the present invention;

FIG. 2A shows in schematic, cross-sectional form an aspect of an embodiment of a slider mechanism for moving a shield into a cleaning chamber according to an aspect of an embodiment of the present invention;

FIG. 3 shows a schematic, side view of an aspect of another possible embodiment of a shield system for protecting a plasma chamber optical element from plasma source material debris;

FIG. 3A shows a schematic, side view of a plurality of hollow tubes illustrating the path of an exemplary light ray through a hollow tube and the path of an exemplary debris particle being captured by a hollow tube;

FIG. 3B shows a schematic, side view of an aspect of another possible embodiment of a shield system for protecting a plasma chamber optical element from plasma source material debris having one bent hollow tube;

FIG. 3C shows a schematic, side view of an aspect of another possible embodiment of a shield system for protecting a plasma chamber optical element from plasma source material debris having one straight hollow tube; and

FIG. 4 shows a schematic, side view of an aspect of another possible embodiment of a shield system for protecting a plasma chamber optical element from plasma source material debris having a heater for heating a light transmitting shield.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Turning now to FIG. 1 there is shown a schematic view of an exemplary production EUV light source, e.g., a laser produced plasma EUV light source 20 according to an aspect of the present invention. Although aspects of the present invention are illustrated with reference to a laser produced plasma (LPP), it is to be appreciated that the present invention is equally applicable to other types of light sources which produce a plasma including an electric discharge produced plasma (“DPP”), a representative construction of which is disclosed in co-owned U.S. Pat. No. 6,815,700 which is hereby incorporated by reference.

Continuing with FIG. 1, an LPP light source 20 may contain a pulsed laser system 22, e.g., a gas discharge excimer or molecular fluorine laser operating at high power and high pulse repetition rate and may be a MOPA configured laser system, e.g., as shown in U.S. Pat. Nos. 6,625,191, 6,549,551, and 6,567,450. The light source 20 may also include a target delivery system 24, e.g., delivering targets in the form of liquid droplets, a liquid stream, solid particles or clusters, solid particles contained within liquid droplets or solid particles contained within a liquid stream. The targets may be delivered by the target delivery system 24, e.g., into the interior of a chamber 26 to a plasma formation site 28, otherwise known as an ignition site or the sight of the “fire ball”.

Laser pulses may be delivered from the pulsed laser system 22 along a laser optical axis 55 through a laser input window 57 in the chamber 26 to the irradiation site, suitably focused, to create an ignition or fire ball that forms a plasma, having certain characteristics, including wavelength of the EUV light produced, type and amount of debris released from the plasma during or after ignition, according to the material of the target.

The light source may also include a collector 30. e.g., a reflector, e.g., in the form of a truncated ellipse, with an aperture to allow the laser light to pass through and reach the ignition site 28. The collector 30 may be, e.g., an elliptical mirror that has a first focus at the ignition site 28 and a second focus at a so-called intermediate point 40 (also called the intermediate focus 40) where the EUV light is output from the light source and input to, e.g., an integrated circuit lithography tool (not shown). The system 20 may also include a target position detection system 42.

The pulsed system 22 may include, e.g., a master oscillator-power amplifier (“MOPA”) configured dual chambered gas discharge laser system having, e.g., an oscillator laser system 44 and an amplifier laser system 48, with, e.g., a magnetic reactor-switched pulse compression and timing circuit 50 for the oscillator laser system 44 and a magnetic reactor-switched pulse compression and timing circuit 52 for the amplifier laser system 48, along with a pulse power timing monitoring system 54 for the oscillator laser system 44 and a pulse power timing monitoring system 56 for the amplifier laser system 48. The system 20 may also include an EUV light source controller system 60, which may also include, e.g., a target position detection feedback system 62 and a firing control system 65, along with, e.g., a laser beam positioning system 66.

The target position detection system may include a plurality of droplet imagers 70, 72 and 74 that provide input relative to the position of a target droplet, e.g., relative to the ignition site and provide these inputs to the target position detection feedback system, which can, e.g., compute a target position and trajectory, from which a target error can be computed, if not on a droplet by droplet basis then on average, which is then provided as an input to the system controller 60, which can, e.g., provide a laser position, direction and timing correction signal, e.g., to the laser beam positioning system 66 that the laser beam positioning system can use, e.g., to control the laser timing circuit and/or to control the laser position and direction changer 68, e.g., to change the focus point of the laser beam to a different ignition point 28.

The imager 74 may, e.g., be aimed along an imaging line 75, e.g., aligned with a desired trajectory path of a target droplet 94 from the target delivery mechanism 92 to the desired ignition site 28 and the imagers 72 and 76 may, e.g., be aimed along respective intersecting imaging lines 76 and 78 that intersect, e.g., along the desired trajectory path at some point along the trajectory path before the desired ignition site 28.

The target delivery control system 90, in response to a signal from the system controller 60 may, e.g., modify the release point of the target droplets 94 as released by the target delivery mechanism 92 to correct for errors in the target droplets arriving at the desired ignition site 28. An EUV light source detector 100 positioned at or near the intermediate focus 40 may also provide feedback to the system controller 60 that can be, e.g., indicative of the errors in such things as the timing and focus of the laser pulses to properly intercept the target droplets in the right place and time for effective and efficient EUV light production.

As shown schematically in FIG. 1 and described in more detail below, an aspect of an embodiment of the present invention includes shielding systems 102 a-e for protecting respective plasma chamber optical element surfaces 104 a-e from debris generated at the plasma formation site 28. In particular, as shown, shield system 102 a protects surface 104 a of laser input window 57, shield systems 102 b, 102 c and 102 d protect respective surfaces 104 b, 104 c and 104 d of respective imaging windows of imagers 70, 72 74 and shield system 102 e protects surface 104 e of EUV light source detector 100.

FIG. 2 shows a system, illustrated schematically and designated generally system 102, for protecting a surface 104 a of an optical element, e.g. laser input window 57, from debris resulting from plasma formation according to an aspect of an embodiment of the present invention. Although the system 102 is shown for protecting a laser input window 57 in FIG. 2, it is to be appreciated that the system 102 shown can be used to protect other optical components including, but not limited to, imaging windows and light detector input surfaces. As shown in FIG. 2, the system 102 may include a cleaning chamber 106 that may be positioned adjacent to the plasma chamber 26.

Discrete shields 108 a,b made of a transparent material, e.g. a material that is transparent to the laser beam 109 (or to light being sampled by an external metrology device, e.g., EUV light or visible light) may be mounted on a mechanism 110 that is configured to sequentially move each shield 108 from the plasma chamber 26 to the cleaning chamber 106 and then back into the plasma chamber 26, and so forth. More specifically, the mechanism can locate each shield 108 in a first position (corresponding to the position shown for shield 108 a) where the shield 108 is interposed between the plasma formation site 28 and the laser input window 57 to prevent debris 112 that is directed toward the laser input window 57 from reaching the window 57. The mechanism 110 can be activated to then move the shield 108 from the first position described above to a second position (corresponding to the position shown for shield 108 b) in which the shield 108 may be disposed within the cleaning chamber 106 to clean deposited debris from the shield 108. Although two shields 108 are shown in FIG. 2, it is to be appreciated that more than two shields 108 and as few as one shield 108 may be used. In addition, a contiguous annularly shaped, strip of transparent material spanning 360 degrees may be used in which only a portion of the strip is interposed between the plasma formation site 28 and the laser input window 57 at any one time. For this case, the interposed portion of the continuous strip constitutes a “shield” for purposes of the present disclosure, and accordingly, the strip constitutes a plurality of shields.

As shown in FIG. 2, the mechanism 110 may include a wheel 114 defining a wheel axis 116. The shield(s) 108 may be attached to the wheel 114, as shown. When activated, a rotation assembly in the mechanism 110 rotates the shield(s) 108 (see arrow 118) about the wheel axis 116 to move the shield 108 from the plasma chamber 26 to the cleaning chamber 106, and after cleaning, back into the plasma chamber 26. This rotation may be performed in increments or may be continuous. With this structural arrangement, one or more shields 108 on the wheel 114 may be cleaned in the cleaning chamber 106, and simultaneously, another shield 108 on the wheel 114 may be used to prevent plasma source debris 112 from reaching the laser input window 57 in the plasma chamber 26.

In the cleaning chamber 106, an etchant source 120 may be provided to establish a concentration of an etchant such as HBr for cleaning debris from the shield 108. Alternatively, suitable etchants can include, but are not necessarily limited to etchants such as Br₂, C1 ₂, HCl, H₂ and HCF₃. For example, an HBr concentration of a few Torr can be used in the cleaning chamber to clean the shields 108. In some cases, a relatively small HBr concentration, e.g. 10 milliTorr may also be maintained in the plasma chamber 46 to etch structures therein. With this arrangement, only a reasonably tight seal between chamber 26 and 106 near the mechanism 110 may be required. In some implementations, an apparatus (not shown) may be included in the cleaning chamber to mechanically clean the shield 108. Alternatively, or in addition to either mechanical cleaning, etching or both, a heat source (not shown) can be used to remove deposited material from the shield 108 (see further discussion below regarding removal of deposit debris by heating). In still another variation, plasma etching or plasma assisted etching may be used in the cleaning chamber 106, e.g., an rf plasma using Argon at a pressure of about 1 mTorr.

FIG. 2A shows another aspect of an embodiment in accordance with the present invention in which the system includes a mechanism 110′ which comprises a slider assembly 122 to move the shield 108 along a linear path (in the direction of arrow 124) from the plasma chamber 26 to the cleaning chamber 106. Although not shown, it is to be appreciated that multiple slider assemblies (each operable on a different shield) may be used allowing the use of one shield 108 in the plasma chamber 26 while one or more shields 108 are simultaneously cleaned in the cleaning chamber 106.

The ability to protect the laser input window 57 from heat and debris as described above may also facilitate the positioning of the window 57 at a location that is closer to the plasma formation site 28 (than a corresponding unprotected window). This, in turn, may be beneficial when the laser input window 57 also functions as a lens. In this case, movement of the lens closer to the plasma formation site may allow the use of a laser source having a reduced laser divergence and pointing stability requirements.

FIG. 3 shows a system according to another aspect of an embodiment of the present invention, illustrated schematically and designated generally system 102′, for protecting a surface 104 d of an optical element, e.g., EUV light detector 100, from debris resulting from plasma formation. As shown, the system 102′ may include a plurality of hollow tubes 126, e.g., so-called capillary tubes, with each tube having a tube wall that surrounds a tube lumen, (i.e., bore). Tubes 126 may be made of a material, e.g., glass, metal or ceramic, e.g., borosilicate material, which reflects EUV light at grazing angles of incidence, e.g., grazing incidence reflection at small (<10 degrees) angles of grazing incidence where the EUV reflectivity of smooth surfaces is relatively high for most materials. As shown, the tubes 126 may be grouped together and housed within a stainless steel housing tube 128 having a similar shape as the tubes 126. In an exemplary embodiment, about 50 bent glass capillary tubes 126 (1 mm outer diameter, 0.78 mm inner diameter, 150 mm long) may be mounted inside of a bent stainless steel tube 128. As shown in FIG. 3A, the tubes 126 may be shaped having a midsection 130 that may be laterally offset from a tube axis 132 defined by the tube ends 134, 136. In particular, the midsection 130 may be offset by a distance 138 that is larger than inner diameter of the tube 126.

FIG. 3A shows that the tubes 126 may be interposed between the plasma formation site 28 and the surface 104 e. FIG. 3A also shows an exemplary path 140 of an EUV light ray and the exemplary path 142 of a debris particle. As shown, the EUV light ray passes through the lumen, (i.e., bore) of a tube 126 after a series of small angle grazing incidence reflections from the inner wall surface of the tube 126 and reaches the surface 104 e. On the other hand, as shown, the debris particle may strike the inner wall of the hollow tube and stick to the inner wall. Moreover, in some cases, the accumulation of debris on the inner wall may result in a smooth enough surface to adequately reflect EUV light at grazing angles of incidence. Use of the tubes 126 may have an advantage over the use of flat mirrors to direct light to a detector in that they will automatically direct the light towards the end of the tube and no complicated alignment is required, like in the case of redirecting mirrors.

FIG. 3 also shows that the system 102′ may include a sub-system 144 for releasing a gas for flow through each tube 126. In each tube 126, the gas may collide with and deflect debris particles into the inner tube wall. The gas then exits each tube 126 enters the plasma chamber. Exemplary gases for this purpose may include gases such as argon and helium which are relatively transparent to EUV light. As shown, the gas may be introduced into the stainless steel tube 128 and the detector 100. Detector 100 may include one or more thin EUV filter foils 146, a multi-layer mirror 148 and a photodiode detector 150, as shown.

The system 102′ may also include a partially transmitting, (e.g., approximately 50% transmitting) plate 152 having small straight channels (several μm diameter, so-called collimated holes) located between the tubes 126 and the detector 100 to block remaining debris particles from hitting the detector 100. For example, the channel plate can be formed of a glass capillary array having a length of about 1-5 mm and capillary diameters between about 5 μm and 100 μm). Also, the channel plate 152 can also be mounted in the stainless steel tube 128 at an angle, θ, e.g. 5-10 degrees, to avoid a straight passage through the channels and ensure grazing-incidence reflection.

FIG. 3B shows another aspect of an embodiment of the present invention having a single bent, hollow tube 126′, for protecting a surface of an optical element, e.g. EUV light detector 100, from debris resulting from plasma formation. FIG. 3C shows yet another aspect of an embodiment of the present invention having a single straight, hollow tube 126″, for protecting a surface of an optical element, e.g. EUV light detector 100, from debris resulting from plasma formation. Long straight tubes 126″ may be used to confine the detector viewing angle. The single tubes 126′ and 126″ can be combined with a gas flow from a gas sub system 144 to increase the probability of collisions with debris particles and may include a partially transmitting plate 152. It is to be appreciated that a plurality of straight tubes 126″ could be bundle together in a manner similar to the bundled bent tubes 126 shown in FIG. 3.

FIG. 4 shows a system according to another aspect of an embodiment of the present invention, designated generally system 102″, for protecting a surface of an optical element, e.g., a laser input window, imager window or EUV light detector, from debris resulting from plasma formation. As shown, the system 102″ may include a shield 108′ made of a light transmissive material, e.g., a material that is substantially transparent to an LPP laser beam (or to light being sampled by an external metrology device, e.g., EUV light or visible light), e.g., CaF₂, fused silica or MgF₂.

In use, the shield 108′ is positioned inside the plasma chamber 26 (see FIG. 1) and located between the plasma formation site 28 and an optical element to thereby allow debris to temporarily deposit on the surface of the shield 108′. The system 102″ may further include a heater 154 to heat the shield 108′ to a temperature sufficient to remove at least a portion the deposited debris, e.g., to remove portions or all of one or more species.

For example, the heater may heat the shield 108′ to a temperature sufficient vaporize at least a portion of a deposited material. For a plasma source material which comprises Li, the heater 154 may be designed to heat the shield 108′ to a temperature in the range of about 400 to 550 degrees C. to vaporize Li from the shield surface.

In some cases, the heater may heat the shield 108′ to a temperature sufficient to initiate a chemical reaction between a deposited material and an etchant gas that is introduced into the plasma chamber. For a plasma source material which comprises Sn, the heater 154 may be designed to heat the shield 108′ to a temperature in the range of about 200 to 325 degrees C. to initiate a reaction between Sn deposits and gaseous etchants, e.g., HBr, to create a reaction product that is then removed from the shield surface, e.g., by evaporation or wicking, e.g. using copper wire.

As shown in FIG. 4, the heater may comprise a band 156, made of a conductive material, e.g., Mo, that is positioned in contact with the shield 108′ and a current source for passing a current through the conductive band 156. The band 156 may also be formed with extensions 158 allowing the band 156 to hold and mechanically support the shield 108′. Other means of heating the shield 108′ include, but are not limited to radiative heaters, microwave heaters, rf heaters and combinations thereof.

It will be understood by those skilled in the art that the aspects of embodiments of the present invention disclosed above are intended to be preferred embodiments only and not to limit the disclosure of the present invention(s) in any way and particularly not to a specific preferred embodiment alone. Many changes and modification can be made to the disclosed aspects of embodiments of the disclosed invention(s) that will be understood and appreciated by those skilled in the art. The appended claims are intended in scope and meaning to cover not only the disclosed aspects of embodiments of the present invention(s) but also such equivalents and other modifications and changes that would be apparent to those skilled in the art. 

1. A system for protecting an EUV light source plasma production chamber optical element surface from debris generated by plasma formation, the system comprising: a shield comprising at least one hollow tube having a tube wall that surrounds a tube lumen, the tube being interposed between the plasma formation site and the optical element and oriented to prevent at least a portion of the debris directed toward the optical element surface from reaching the surface and allowing at least a portion of light generated at the plasma formation site to pass through the lumen and reach the optical element surface.
 2. The system of claim 1 further comprising: wherein the shield comprises a plurality of hollow tubes, with each tube having a respective lumen and wherein each tube is oriented in the plasma chamber to allow light generated at the plasma formation site to pass through the respective tube lumen and reach the optical element surface.
 3. The system of claim 1 further comprising: wherein the hollow tube has a first end and a second end, a lumen diameter, d; and defines a linear tube axis from the first end to the second end and wherein the tube is formed with a midsection between the first end and second end, the midsection being laterally offset from the tube axis by an offset distance, D, with D≧d.
 4. The system of claim 2 further comprising: wherein each hollow tube has a respective first end and a respective second end, a respective lumen diameter, d; and defines a respective linear tube axis from the respective first end to the respective second end and wherein each tube is formed with a respective midsection between the respective first end and respective second end, the respective midsection being laterally offset from the respective tube axis by an offset distance, D, with D≧d.
 5. The system of claim 1 further comprising: wherein the tube is a straight tube.
 6. The system of claim 1 further comprising: wherein the hollow tube is made of a material selected from the group of materials consisting of glass, metal and ceramic.
 7. The system of claim 1 further comprising: a sub-system for passing a gas through the tube to deflect debris into the tube wall.
 8. The system of claim 1 further comprising: a plate formed with a plurality of through holes, the plate positioned between the tube and the optical element.
 9. The system of claim 1 further comprising: wherein a plasma is generated at the plasma formation site by an electrical discharge between a pair of electrodes.
 10. The system of claim 1 further comprising: wherein a plasma is generated at the plasma formation site by irradiating a target material with a laser beam.
 11. The system of claim 1 further comprising: wherein the plasma chamber optical element surface is selected from the group of surfaces consisting of a surface of a light detector and a surface of an imaging window.
 12. A system for protecting an EUV light source plasma production chamber optical element surface from debris generated by plasma formation, the system comprising: a hollow element surrounding a passageway, the passageway establishing fluid communication between the plasma formation site and the optical element to allow light generated at the plasma formation site to reach the optical element surface; and a sub-system for passing a gas through the passageway to deflect debris into the hollow element.
 13. The system of claim 12 further comprising: wherein the element comprises at least one tube.
 14. The system of claim 13 further comprising: wherein each tube has a respective first end and a respective second end, a respective lumen diameter, d; and defines a respective linear tube axis from the respective first end to the respective second end and wherein each tube is formed with a respective midsection between the respective first end and respective second end, the respective midsection being laterally offset from the respective tube axis by an offset distance, D, with D≧d.
 15. The system of claim 12 further comprising: wherein the tube is a straight tube.
 16. A method for protecting an optical element surface from debris generated by plasma formation, the method comprising the steps of: interposing a shield between the plasma formation site and the optical element, the shield comprising at least one hollow tube having a tube wall that surrounds a tube lumen, the tube being oriented to prevent at least a portion of the debris directed toward the optical element surface from reaching the surface and allowing at least a portion of light generated at the plasma formation site to pass through the lumen and reach the optical element surface.
 17. A method as recited in claim 16 wherein each hollow tube has a first end and a second end, a lumen diameter, d; and defines a linear tube axis from the first end to the second end and wherein each tube is formed with a midsection between the first end and second end, the midsection being laterally offset from the tube axis by an offset distance, D, with D≧d.
 18. A method as recited in claim 16 wherein each tube is a straight tube.
 19. A method as recited in claim 16 further comprising the step of: passing a gas through the tube to deflect debris into the tube wall.
 20. A method as recited in claim 16 further comprising the step of: positioning a plate between the tube and the optical element, the plate formed with a plurality of through holes. 